1. Field of the Invention
The present invention relates to the localization of positions of complex-shaped organs exerting complex movements one with respect to the other.
More particularly, the present invention aims, for such organs, at locating points of an organ exerting specific movements with respect to another organ, for example, points of an organ whose distance remains invariant during the movement of this organ with respect to another organ.
The invention can be used in complex mechanical systems in which it is practically impossible to determine by calculation the movement of some organs with respect to other organs. More particularly, the invention applies in case of physiologic organs and will be described hereinafter in the surgical field, and more particularly the orthopaedic surgery of the knee.
2. Discussion of the Related Art
There are clinical situations in which it is necessary to link two organs, while taking into account certain medical constraints. This particularly applies for surgery of the knee, and more precisely during repair of one or more crossed ligaments injured by a trauma.
This surgical operation consists in replacing one or several ligaments by an implant taken from the patient (generally from his kneecap tendon) or by an artificial ligament. To be efficient, this implant must be at the limit of stress during all normal movements (flexion-extension-rotation) of the knee. If the ligament is too tightened, it will rapidly break; if the ligament is too loose, it will not disallow abnormal movements. Thus, the medical constraint can be translated into an isometry constraint: the ligament must maintain a constant length during flexion-extension movements.
To achieve this purpose, the ligament must be positioned in such a way that the distance between its fixation points on the femur and tibia remains constant. To determine these points, the surgeon has a knowledge about anatomy and physiology that can be complemented by various devices. The tibial point is theoretically easy to see, both in conventional surgery and in arthroscopy because it is clearly delineated, and the surgeon knows where it is positioned with respect to the front edge of the top portion of the tibial notch. It has been demonstrated that a small variation in its position little affects the ligament's isometry. In contrast, the femoral point is positioned very deep in the trochlear throat; so, the surgeon can only roughly visually determine its position. In practice, it has been evidenced that a small variation in the position of the femoral point could substantially impair the ligament's isometry. Accordingly, to guide the selection of the femoral point, it has been suggested to position a spring and to check by flexion-extension movements whether the fixation points where this spring is fastened maintain the isometry (and therefore the isometric stress) of the spring. Once the fixation points are positioned, one must be able of sighting them. Indeed, to be strongly fixed, the ligament is introduced into femoral and tibial tunnels. The realization of the tibial tunnel does not raise any particular problem, both in conventional surgery as in arthroscopy. The femoral tunnel can be carried out in two manners: either from the outside to the inside, or from the inside to the outside (blind tunnel). In the first case (from the outside to the inside), the drill pierces the external cortical of the femur while being guided by a view finder--that is not always be easily used and not always as precise as desirable--towards the fixation point, at the intercondyle notch. The drill must be precisely positioned to arrive at the desired fixation point, any inadequate positioning having, as indicated above, detrimental consequences on the ligament's isometry. In the second case (from the inside to the outside), the drill penetrates the notch at the isometric point.
The difficulties encountered in order to precisely determine the fixation point in the femur, as well as to be absolutely sure of the precision of the drilling technique from outside to the inside of the femur tunnel, account for the difficulty of this surgical operation, both in conventional surgery and in arthroscopy. In practice, such a surgical operation:
is frequent: several thousand cases each year in France because of the importance of the functional impairment induced by the injury of the front crossed ligament; such an injury is frequent since it is caused by traumas occurring during practice of common sports (ski, football, handball, volleyball, etc.); PA1 aims at reducing the percentage of degenerative arthrosis of knees injured by a trauma; it is admitted that, with the conventional techniques involving ligament surgery, 50% of the injured knees, that had an operation or not, will need, 25 years after the injury, a whole knee prosthesis.
The frequency and the importance of such a pathology justify the improvement of the operation technique, a key point being the improvement of the isometry of the implanted ligament.
A general object of the invention is to provide a method and a system for determining an invariant point of an organ movable with respect to another organ.
A more specific object of the present invention is to determine the position of the fixation point of a crossed ligament in the femur.
A further object of the invention is to provide a method for intervening at the previously determined fixation point.